Subsea Wellhead System With Hydraulically Set Seal Assemblies

ABSTRACT

A wellhead system, the system including a wellhead housing having a central axis, an upper end, and a radially inner surface extending axially from the upper end. The system also includes a first casing string disposed within the wellhead housing. The first casing string includes a casing hanger and casing extending axially from the casing hanger. The system further includes a first ram block movably coupled to the housing and a second ram block movably coupled to the housing and radially opposed to the first ram block. Each ram block has a withdrawn position radially spaced apart from the casing hanger of the first casing string and an advanced position sealingly engaging the casing hanger of the first casing string.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from U.S. Provisional Application Ser. No. 61/776,015, filed Mar. 11, 2013.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

The invention relates generally to subsea oil and gas operations. More particularly, the invention relates to subsea wellhead assemblies. Still more particularly, the present invention relates to hydraulically actuated seal assemblies to seal and secure casing hangers within subsea wellheads.

In offshore drilling operations, a large diameter hole is drilled to a selected depth in the sea bed. Then, a primary conductor secured to the lower end of an outer wellhead housing, also referred to as a low pressure housing, is run into the borehole with the outer wellhead housing positioned at the sea floor. To secure the primary conductor and outer wellhead housing in place, cement is pumped down the primary conductor and allowed to flow back up the annulus between the primary conductor and the borehole sidewall. A drill bit is then lowered through the primary conductor to drill the borehole to a second depth. Next, an inner wellhead housing, also referred to as a high pressure housing, is seated in the upper end of the outer wellhead housing. A string of casing secured to the lower end of the inner wellhead housing or seated in the inner wellhead housing extends downward through the primary conductor. Cement is pumped down the casing string, and allowed to flow back up the annulus between the casing string and the primary conductor. Drilling continues while successively installing concentric casing strings of varying diameters that line the borehole.

Typically, each casing string is radially and axially supported with a casing/casing hanger provided at its upper end. Each hanger is seated in a mating profile provided within the inner wellhead housing, and the remainder of the casing string extends downhole therefrom. Each successive casing string is cemented in place by pumping cement down the casing string and allowing it to flow back up the annulus between the casing string and the borehole sidewall and/or previously installed casing string. In addition, an annular seal assembly is installed between each casing hanger and the inner wellhead housing.

Casing strings are conventionally installed with a running tool that transports and installs the casing string and an associated seal assembly. The seal assembly is held in position between the casing hanger and the mating landing shoulder as cement is pumped down the casing string and back up the annulus between the casing string and the inner wellhead housing or radially adjacent casing string. Fluids in the annulus (e.g., mud, sea water, etc.) are displaced by the cement and flow upward around the seal assembly. Once the casing string is cemented in place, the annular seal assembly is energized and radially expanded to completely seal off the upper end of the annulus. As the seal assembly is energized, a lock down ring positioned about the casing hanger is expanded into a shallow groove in the inside of the inner wellhead housing. The lock down ring functions to lock down the casing hanger such that the casing string cannot move upward within the inner wellhead housing in response to downhole fluid pressures.

During the running and installation of the casing string and associated seal assembly, the running tool is responsible for numerous functions (e.g., delivery of the casing string and associated seal assembly, installation of the casing string and associated seal assembly, energizing of the seal assembly, etc.). Consequently, the running tool is usually a relatively complex tool that may be prone to malfunction. Further, although the un-energized seal assembly allows the flow of displaced fluids through the upper end of the annulus, it does obstruct the flow of displaced fluids. Such obstruction may increase the time required to cement the casing string and/or result in an increase in the fluid pressure within the annulus, which may, in some cases, damage the formation or the downhole equipment. Moreover, conventional lock down rings usually have a relatively thin cross-section and small size, and thus, have limited load capacities which are ill suited for high pressure wells.

BRIEF SUMMARY OF THE DISCLOSURE

These and other needs in the art are addressed in one embodiment by a wellhead system. In an embodiment, the wellhead system comprises a wellhead housing having a central axis, an upper end, and a radially inner surface extending axially from the upper end. In addition, the wellhead system comprises a first casing string disposed within the wellhead housing. The first casing string includes a casing hanger and casing extending axially from the casing hanger. Further, the wellhead system comprises a first ram block movably coupled to the housing. Still further, the wellhead system comprises a second ram block movably coupled to the housing and radially opposed to the first ram block. Each ram block has a withdrawn position radially spaced apart from the casing hanger of the first casing string and an advanced position sealingly engaging the casing hanger of the first casing string.

These and other needs in the art are addressed in another embodiment by a method of installing one or more casing string within wellhead housing. In an embodiment, the method comprises lowering a first casing string into a wellhead housing having a central axis. The casing string includes a casing hanger and casing extending axially from the casing hanger. In addition, the method comprises moving a first ram block coupled to the wellhead housing radially inward into sealing engagement with the casing hanger. Further, the method comprises moving a second ram block coupled to the wellhead housing radially inward into sealing engagement with the casing hanger.

These and other needs in the art are addressed in another embodiment by a wellhead system. In an embodiment, the wellhead system comprises a wellhead housing having a central axis, an upper end, and a radially inner surface extending axially from the upper end. In addition, the wellhead system comprises a first annular recess extending radially outward from the inner surface. Further, the wellhead system comprises a first ram block disposed in the first annular recess, and a second ram block disposed in the first annular recess axially opposite the first ram block. Still further, the wellhead system comprises a first linear actuator coupled to the wellhead housing and configured to move the first ram block radially toward and away from the second ram block. Moreover, the wellhead system comprises a second linear actuator coupled to the wellhead housing and configured to move the second ram block radially toward and away from the first ram block.

Embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior devices, systems, and methods. The foregoing has outlined rather broadly the features and technical advantages of the invention in order that the detailed description of the invention that follows may be better understood. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the disclosed embodiments, reference will now he made to the accompanying drawings in which:

FIG. 1 is a side cross-sectional view of an embodiment of a wellhead system including a plurality of annular seal assemblies in accordance with the principles disclosed herein;

FIG. 2 is a cross-sectional view of the wellhead system of FIG. 1 taken along section II-II of FIG. 1 and with the illustrated pair of opposed ram blocks in the radially advanced positions;

FIG. 3 is a cross-sectional view of the wellhead system of FIG. 1 taken along section II-II of FIG. 1 and with the illustrated pair of opposed ram blocks in the radially withdrawn positions;

FIG. 4 a is a cross-sectional side view of one pair of opposed ram blocks of the wellhead system of FIG. 1 in the radially withdrawn positions;

FIG. 4 b is a cross-sectional side view of the pair of opposed ram blocks of FIG. 4 a in the radially advanced positions;

FIGS. 5 a and 5 b are cross-sectional side views of an alternative embodiment of a pair of opposed interlocking ram blocks in the radially advanced and withdrawn positions, respectively;

FIGS. 6 a and 6 b are cross-sectional side views of an alternative embodiment of a pair of opposed interlocking ram blocks in the radially advanced and withdrawn positions, respectively; and

FIGS. 7-12 are sequential, side cross-sectional views of the installation of the three concentric casing strings of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following discussion is directed to various exemplary embodiments. However, one skilled in the art will understand that the examples disclosed herein have broad application, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment.

Certain terms are used throughout the following description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness.

In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices, components, and connections. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis.

Referring now to FIG. 1, a subsea wellhead system 100 for producing a subsea wellbore is shown. In this embodiment, system 100 includes an inner wellhead housing 101 seated in the upper end of an outer wellhead housing (not shown), a plurality of concentrically arranged casing strings 120 disposed within wellhead housing 101, a plurality of sealing assemblies 150, and a plurality of linear actuators 170. One sealing assembly 150 is provided for each casing string 120, and thus, since there are three casing strings 120 in this embodiment, there are also three sealing assemblies 150. As will be described in more detail below, each sealing assembly 150 forms an annular seal with wellhead housing 101 and the corresponding casing string 120, thereby preventing fluid flow therebetween. Each sealing assembly 150 is moved into and out of sealing engagement with the corresponding casing string 120 with a pair of actuators 170.

Wellhead housing 101 is a rigid elongate tubular having a central or longitudinal axis 105, a first or upper end 101 a disposed at or proximal the sea floor, a second or lower end (not shown) disposed in the formation below the sea floor, a radially outer surface 102 extending axially from upper end 101 a to the lower end, a radially inner surface 103 extending axially from upper end 101 a to the lower end, and a central throughbore 104 defined by the inner surface 103. Inner surface 103 includes an upward facing frustoconical landing shoulder 106 proximal upper end 101 a and a plurality of axially-spaced annular grooves or recesses 107 axially positioned between landing shoulder 106 and upper end 101 a.

Referring now to FIGS. 1 and 2, each annular recess 107 is defined by a first or upper annular planar surface 107 a, a second or lower annular planar surface 107 b oriented parallel to surface 107 a, and an annular cylindrical surface 107 c extending axially between surface 107 a, b. Surfaces 107 a, b are each disposed in a plane oriented perpendicular to axis 105, and surface 107 c is oriented parallel to axis 105. A pair of circumferentially-spaced bores 108 extend radially from outer surface 102 to each recess 107. In this embodiment, each pair of bores 108 associated with each given recess 107 are angularly-spaced 180° apart about axis 105. An end cap or closure member 109 is disposed over each bore 108 along outer surface 102.

Referring again to FIG. 1, each casing string 120 has a different diameter, but is otherwise the same. In this embodiment, each casing string 120 is a rigid tubular coaxially aligned with axis 105 and having a first or upper end 120 a and a second or lower end (not shown) disposed downhole. Each upper end 120 a comprises a frustoconical landing seat or surface 120 b. In addition, each casing string 120 includes a casing hanger 121 and a plurality of pipe joints coupled together end-to-end to form casing 128 extending downward from hanger 121. Each hanger 121 has a first or upper end 121 a defining end 120 a, a second or lower end 121 b opposite end 121 a and attached to casing 128, a radially outer surface 122 extending axially from end 121 a to end 121 b, a radially inner surface 123 extending axially from end 121 a to end 121 b, and a throughbore 124 defined by inner surface 123. Each casing 128 is a cylindrical tubular with a central throughbore 129 extending axially from lower end 121 a of the corresponding hanger 121 to the lower end of the corresponding string 120. Each throughbore 129 is contiguous with and in fluid communication with the corresponding hanger throughbore 124.

Referring still to FIG. 1, outer surface 122 of each hanger 121 includes an annular recess 125 extending axially from upper end 120 a, an annular downward facing frustoconical shoulder 126 axially positioned between recess 125 and lower end 121 b, and a plurality of circumferentially-spaced grooves 127 extending axially from lower end 121 b to recess 125. Grooves 127 define a plurality of circumferentially-spaced fluid flow passages 127 a along the outside of each hanger 121. In this embodiment, each recess 125 is formed by a cylindrical surface 125 a extending axially from end 120 a to an upward facing frustoconical shoulder 126 b. Shoulder 126 b is oriented at an angle α relative to a radius of vertical axis 105 such that it inclines moving radially inward. Angle α is preferably between 2 and 14°. In this embodiment, angle α is 5°. As will be described in more detail below, shoulder 126 of the radially outermost casing string 120 is seated against mating shoulder 106 of inner wellhead housing 101, and shoulder 126 of each remaining casing string 120 is seated against the mating landing seat 120 b at upper end 120 a of a radially adjacent casing string 120. In addition, as will be explained in more detail below, each recess 125 and shoulder 126 b are configured to engage and interlock with one of the sealing assemblies 150.

Referring now to FIGS. 1-3, each sealing assembly 150 includes a pair of circumferentially-spaced ram blocks 151 moveably disposed in the corresponding annular recess 107, a first or upper annular seal assembly 160′ axially positioned between the pair of ram blocks 151 and wellhead housing 101, and a second or lower annular seal assembly 160″ axially positioned between the pair of ram blocks 151 and wellhead housing 101.

As best shown in FIG. 1, each pair of ram blocks 151 slidably engages surfaces 107 a, b of the corresponding recess 107 and are angularly spaced 180° about axis 105. In other words, each pair of ram blocks 151 are positioned radially opposite each other. In this embodiment, each ram block 151 comprises a rigid semi-circular body 152 having a planar upper surface 153, a planar lower surface 154 a oriented parallel to surface 153, a frustoconical engagement surface 154 b extending from the lower planar surface 154 a, a radially outer semi-cylindrical surface 155 extending axially between surfaces 153, 154 a, a radially inner semi-cylindrical surface 156 extending axially between surfaces 153, 154 b, and a pair of end surfaces 157 extending radially between surfaces 155, 156. Engagement surface 154 b is oriented an angle β relative to a radius of the vertical axis 105. Angle β is preferably between 2 and 14°. In this embodiment, angle β is preferably equal to the angle α, previously described. Thus, in this embodiment angle β is 5°. However, it should be noted that in other embodiments, angle β may not be equal to angle α while still complying with the principles disclosed herein. As will be described in more detail below, engagement surface 154 b is configured to engage with shoulder 126 b of recess 125. Upper and lower seal members 162 sealingly engage surfaces 107 a, b, respectively, of the corresponding recess 107. A seal element 158 is mounted to each inner surface 156 and extends circumferentially between end surfaces 157.

Still referring to FIG. 1, in this embodiment, each upper seal assembly 160′ includes an annular recess or seal gland 161 in upper surface 153 of the corresponding ram block 151 and an annular seal member 162 seated in gland 161; and each lower seal assembly 160″ includes an annular recess or seal gland 161 in lower surface 154 a of the corresponding ram block 151 and an annular seal member 162 seated in gland 161. Each seal member 162 forms an annular sliding or dynamic seal with inner wellhead housing 101 and an annular static seal with the corresponding pair of ram blocks 151.

As will be described in more detail below, one linear actuator 170 is coupled to each ram block 151 and is configured to move the associated ram block 151 radially (inward and outward) between a radially withdrawn position spaced apart from a corresponding casing hanger 121 and a radially advanced position sealingly engaging the corresponding casing hanger 121. In the advanced positions shown in FIG. 2, opposed end surfaces 157 are brought into engagement and seal elements 158 are brought into sealing engagement with a corresponding casing hanger 121; and in the withdrawn positions shown in FIG. 3, opposed end surfaces 157 are spaced apart and seal elements 158 are spaced from the corresponding casing hanger 121.

Referring now to FIGS. 4 a and 4 b, each pair of opposed end surfaces 157 have interlocking mating profiles that function to prevent end surfaces 157 from being urged axially apart and prevent fluid flow therebetween when ram blocks 151 are in the radially advanced positions. For purposes of clarity and further explanation, in FIGS. 4 a and 4 b, opposed end surfaces 157 are designated with reference numerals 157 a, 157 b. In this embodiment, end surface 157 a includes a recess 159 a extending between inner and outer surfaces 155, 156, and opposed end surface 157 b includes a projection or lip 159 b extending between inner and outer surfaces 155, 156. Projection 159 b is sized, shaped, and positioned to mate and engage with recess 159 a when opposed surfaces 157 a, 157 b are brought together (i.e., when the corresponding ram blocks 151 are in the radially advanced positions). End faces 157 a, 157 b are preferably lined or provided with seal elements (e.g., elastomeric seal elements) that form seals therebetween when end faces 157 a, 157 b are brought into interlocking engagement. Although only one pair of opposed end surfaces 157 of one set of opposed ram blocks 151 are shown in FIGS. 4 a and 4 b, it is to be understood that the other pairs of opposed end surfaces 157 in wellhead system 100 are the same.

In FIGS. 4 a and 4 b, recess 159 a and projection 159 b have mating semi-cylindrical geometries. However, in other embodiments, the recess (e.g., recess 160) and the projection (e.g., projection 161) of opposed end surfaces (e.g., end surfaces 157 a, 157 b) can have other suitable mating geometries including, without limitation, triangular as shown in FIGS. 5 a and 5 b, rectangular as shown in FIGS. 6 a and 6 b.

Referring again to FIGS. 1-3, each linear actuator 170 is coupled to one ram block 151 and configured to move that ram block 151 between the radially withdrawn and advanced positions. In this embodiment, each linear actuator 170 is the same. In particular, each linear actuator 170 is coupled to inner wellhead housing 101 and includes a cylinder 171 mounted to one end cap 109, a piston 172 slidably disposed within cylinder 171, and a rigid connecting shaft or rod 173 extending radially from piston 172 through end cap 109 and bore 108 to a corresponding ram block 151. A seal assembly is included about rod 173, such that the flow of fluids along rod 173 is obstructed. Each piston 172 sealingly engages the corresponding cylinder 171 and divides the inside of that cylinder 171 into a first or radially outer chamber 174 a and a second or radially inner chamber 174 b. By controlling the fluid pressure within chambers 174 a, b, the corresponding piston 172 can be drive to move radially inward or radially outward within the cylinder 171. Thus, by increasing the fluid pressure in chamber 174 a relative to chamber 174 b, piston 172 is moved radially inward, and by decreasing the fluid pressure in chamber 174 a relative to chamber 174 b, piston 172 is moved radially outward. Each piston 172 is coupled to one ram block 151 with one connecting rod 173, and thus, by moving pistons 172 radially inward and outward, the corresponding ram blocs 151 are moved between the withdrawn and advanced positions. In this embodiment, linear actuators 170 are hydraulic piston-cylinder assemblies, and thus, pressurized hydraulic fluid is used to move pistons 172 within cylinders 171. Such pressurized hydraulic fluid can be supplied by any suitable means known in the art including, without limitation, with a subsea accumulator, a hydraulic pump, a hydraulic fluid supply line, or combinations thereof.

Referring now to FIG. 1, as previously described, casing strings 120 are concentrically arranged one-inside-the-other. For purposes of clarity and further explanation, the radially outermost casing string 120 is designated with reference numeral 120′, the radially intermediate casing string 120 is designated with reference numeral 120″, and the radially innermost casing string 120 is designated with reference numeral 120′″. In addition, the axially lowermost ram blocks 151 associated with casing hanger 121 of casing string 120′ are designated with reference numerals 151′, the axially intermediate ram blocks 151 associated with casing hanger 121 of casing string 120″ are designated with reference numerals 151″, and the axially uppermost ram blocks 151 associated with casing hanger 121 of casing string 120′″ are designated with reference numerals 151′″.

An annulus 190′ is radially disposed between casing 128 of string 120′ and inner wellhead housing 101, an annulus 190″ is radially disposed between casing 128 of string 120″ and casing string 120′, and an annulus 190′″ is radially disposed between casing 128 of string 120′″ and casing string 120″. Annulus 190′ is in fluid communication with passages 127 a disposed about casing hanger 121 of casing string 120′, annulus 190″ is in fluid communication with passages 127 a disposed about casing hanger 121 of casing string 120″, and annulus 190′″ is fluid communication with passages 127 a disposed about casing hanger 121 of casing string 120′″. When ram blocks 151′ are radially withdrawn, fluids in annulus 190′ are free to flow through recesses 127 and recess 125 in casing hanger 121 of casing string 120′; when ram blocks 151″ are radially withdrawn, fluids in annulus 190″ are free to flow through recesses 127 and recess 125 in casing hanger 121 of casing string 120″; and when ram blocks 151′″ are radially withdrawn, fluids in annulus 190′″ are free to flow through recesses 127 and recess 125 in casing hanger 121 of casing string 120′″. In other words, when ram blocks 151′ are radially withdrawn, they do not obstruct the flow of fluids from annulus 190′ through passages 127 a and recess 125 in casing hanger 121 of casing string 120′; when ram blocks 151″ are radially withdrawn, they do not obstruct the flow of fluids from annulus 190″ through passages 127 a and recess 125 in casing hanger 121 of casing string 120″; and when ram blocks 151′″ are radially withdrawn, they do not obstruct the flow of fluids from annulus 190′″ through passages 127 a and recess 125 in casing hanger 121 of casing string 120′″. When ram blocks 151′, 151″, 151′″ are radially withdrawn, fluid flow through annular recesses 107 (between wellhead housing 101 and rams 151′, 151″, 151′″ ) is prevented by seal assemblies 160′, 160″. On the other hand, when ram blocks 151′ are radially advanced, ram blocks 151′, sealing engagement of seal elements 158 and casing hanger 121 of casting string 120′, sealing engagement of opposed end faces 157 of ram blocks 151′, and corresponding seal assemblies 160′, 160″ prevent fluid flow from annulus 190′ through recesses 127 and recess 125 in casing hanger 121 of casing string 120′; when ram blocks 151″ are radially advanced, ram blocks 151″, sealing engagement of seal elements 158 and casing hanger 121 of casting string 120″, sealing engagement of opposed end faces 157 of ram blocks 151″, and corresponding seal assemblies 160′, 160″ prevent fluid flow from annulus 190″ through recesses 127 and recess 125 in casing hanger 121 of casing string 120″; and when ram blocks 151′″ are radially advanced, ram blocks 151′″, sealing engagement of seal elements 158 and casing banger 121 of casting string 120′″, sealing engagement of opposed end faces 157 of ram blocks 151′″, and corresponding seal assemblies 160′, 160″ prevent fluid flow from annulus 190′″ through recesses 127 and recess 125 in casing hanger 121 of casing string 120′″.

Referring now to FIGS. 7-12, the installation of casing strings 120 within inner wellhead housing 101 is shown, in general, casing strings 120 are lowered and set in wellhead housing 101 one at a time. In FIGS. 7 and 8, radially outermost casing string 120′ is shown being set in wellhead housing 101; in FIGS. 9 and 10, radially intermediate casing string 120″ is shown being set in wellhead housing 101 after setting casing string 120′; and in FIGS. 11 and 12, radially innermost casing string 120′″ is shown being set in wellhead housing 101 after setting casing string 120″.

Referring first to FIG. 7, all ram blocks 151 are in their withdrawn positions and casing string 120′ is lowered into inner housing 101 with a running tool (not shown) until annular shoulder 126 of hanger 121 axially abuts mating shoulder 106 in housing 101. Next, cement 195 is pumped down casing string 120′ (down throughbores 124, 129) and back up annulus 190′ between casing string 120′ and wellhead housing 101. During the cementing process, fluids disposed within casing string 120′ and annulus 190′ are displaced by cement 195 and flow up through annulus 190′, and then exit the upper end of annulus 190′ through grooves 127 and recess 125 of casing hanger 121 of casing string 120′. Ram blocks 151 are radially withdrawn into recesses 107, and thus, do not obstruct or interfere with the flow of displaced fluids exiting annulus 190′. Moving now to FIG. 8, one annulus 190′ is sufficiently filled with cement 195, the opposed ram blocks 151 associated with casing hanger 121 of casing string 120′ are moved with corresponding actuators 170 from the withdrawn positions to the advanced positions with elements 158 sealingly engaging surface 125 a and shoulder 126 b and opposed ends surfaces 157 interlocked and sealingly engaging each other (not shown in FIG. 8). Because the shoulder 126 b of recess 125 and the engagement surface 154 b of each ram block 151 are inclined, as ram blocks 151 engage and squeeze against shoulder 126 b and surface 125 a, casing hanger 121 (and hence casing string 120′) is urged axially downward to sufficiently seat shoulder 126 against mating shoulder 106. Radial compression of ram blocks 151 against inclined shoulder 126 b and surface 125 a also resists and/or prevents casing string 120′ from moving axially upward relative to inner wellhead housing 101 in response to, for example, fluid pressure or thermal expansion. Although ram blocks 151 corresponding to casing hanger 121 of string 120′ are radially advanced into engagement with casing hanger 121 to set and secure string 120′, the remaining ram blocks 151 (i.e., ram blocks associated with casing hangers 120 of strings 120″, 120′″) are in their radially withdrawn positions.

Referring now to FIG. 9, after outer casing string 120′ has been installed, cemented, and securely set within the housing 101, intermediate casing string 120″ is installed in a similar manner. In particular, ram blocks 151 associated with casing hangers 121 of strings 120″, 120′″ are in their withdrawn positions and casing string 120″ is lowered into inner housing 101 with a running tool (not shown) until annular shoulder 126 of hanger 121 axially abuts mating landing seat 120 b at upper end 120 a of string 120′. Next, cement 195 is pumped down casing string 120″ (down throughbores 124, 129) and back up annulus 190″ between casing strings 120′, 120″. During the cementing process, fluids disposed within casing string 120″ and annulus 190″ are displaced by cement 195 and flow up through annulus 190″, and then exit the upper end of annulus 190″ through grooves 127 and recess 125 of casing hanger 121 of casing string 120″. Ram blocks 151 associated with casing hangers 121 of strings 120″, 120′″ are radially withdrawn into recesses 107, and thus, do not obstruct or interfere with the flow of displaced fluids exiting annulus 190″. Moving now to FIG. 10, once annulus 190″ is sufficiently filled with cement 195, the opposed ram blocks 151 associated with casing hanger 121 of casing string 120″ are moved with corresponding actuators 170 from the withdrawn positions to the advanced positions with elements 158 sealingly engaging surface 125 a and shoulder 126 b and opposed ends surfaces 157 interlocked and sealingly engaging each other (not shown in FIG. 10). Because the shoulder 126 b of recess 125 and the engagement surface 154 b of each ram block 151 are inclined, as ram blocks 151 engage and squeeze against shoulder 126 b and surface 125 a, casing hanger 121 (and hence casing string 120″) is urged axially downward to sufficiently seat shoulder 126 against landing seat 120 b of string 120′. Radial compression of ram blocks 151 against inclined shoulder 126 b and surface 125 a also resists and/or prevents casing string 120″ from moving axially upward relative to inner wellhead housing 101 in response to, for example, fluid pressure or thermal expansion. Although ram blocks 151 corresponding to casing hanger 121 of string 120″ are radially advanced into engagement with casing hanger 121 to set and secure string 120″, the remaining ram blocks 151 (i.e., ram blocks associated with casing hanger 120 of string 120′″) are in their radially withdrawn positions.

Referring now to FIG. 11, after intermediate casing string 120″ has been installed, cemented, and securely set within the housing 101, inner casing string 120′″ is installed in a similar manner. In particular, ram blocks 151 associated with casing hangers 121 of string 120′″ are in their withdrawn positions and casing string 120′″ is lowered into inner housing 101 with a running tool (not shown) until annular shoulder 126 of hanger 121 axially abuts mating landing seat 120 b at upper end 120 a of string 120″. Next, cement 195 is pumped down casing string 120′″ (down throughbores 124, 129) and back up annulus 190′″ between casing strings 120″, 120′″. During the cementing process, fluids disposed within casing string 120′ and annulus 190′″ are displaced by cement 195 and flow up through annulus 190′″, and then exit the upper end of annulus 190′″ through grooves 127 and recess 125 of casing hanger 121 of casing string 120′″. Ram blocks 151 associated with casing hanger 121 of string 120′″ are radially withdrawn into recesses 107, and thus, do not obstruct or interfere with the flow of displaced fluids exiting annulus 190′″. Moving now to FIG. 12, once annulus 190′″ is sufficiently filled with cement 195, the opposed ram blocks 151 associated with casing hanger 121 of casing string 120′″ are moved with corresponding actuators 170 from the withdrawn positions to the advanced positions with elements 158 sealingly engaging surface 125 a and shoulder 126 b and opposed ends surfaces 157 interlocked and sealingly engaging each other (not shown in FIG. 12). Because the shoulder 126 b of recess 125 and the engagement surface 154 b of each ram block 151 are inclined, as ram blocks 151 engage and squeeze against shoulder 126 b and surface 125 a, casing hanger 121 (and hence casing string 120′″) is urged axially downward to sufficiently seat shoulder 126 against mating landing seat 120 b. Radial compression of ram blocks 151 against inclined shoulder 126 b and surface 125 a also resists and/or prevents casing string 120′″ from moving axially upward relative to inner wellhead housing 101 in response to, for example, fluid pressure or thermal expansion.

In the manner described, a plurality of concentric casing strings 120 are installed and set within inner wellhead housing 101. Since sealing assemblies 150 are housed in wellhead housing 101 and energized by actuators 170, they do not need to be delivered subsea, held, or installed with the running tool that installs strings 120. This offers the potential for a less complex running tool that is less prone to malfunctions. Moreover, since ram blocks 151 corresponding to each string 120 are radially withdrawn during installation and cementing of that string 120, they do not present any obstruction to the flow of displaces fluids exiting the annulus around that string 120, thereby lowering the risk of over pressurizing the wellbore and compromising wellbore integrity during cementing. Still further, as compared to conventional lock down rings, ram blocks 151 offer the potential for a more robust, secure means to set and hold down strings 120, thereby offering the potential for enhanced load capacity more suited for use with medium and high pressure

While preferred embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of the invention. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simply subsequent reference to such steps. 

What is claimed is:
 1. A wellhead system, comprising: a wellhead housing having a central axis, an upper end, and a radially inner surface extending axially from the upper end; a first casing string disposed within the wellhead housing, wherein the first casing string includes a casing hanger and casing extending axially from the casing hanger; a first ram block movably coupled to the housing; a second ram block movably coupled to the housing and radially opposed to the first ram block; wherein each ram block has a withdrawn position radially spaced apart from the casing hanger of the first casing string and an advanced position sealingly engaging the casing hanger of the first casing string.
 2. The wellhead system of claim 1, wherein the wellhead housing includes an annular recess extending radially outward from the inner surface of the wellhead housing, and wherein the first and second ram blocks are at least partially disposed within the annular recess.
 3. The wellhead system of 2, further comprising an annular seal assembly positioned between the wellhead housing and each ram block.
 4. The wellhead system of claim 1, wherein the inner surface of the wellhead housing includes a landing shoulder, and wherein the casing hanger is seated against the landing shoulder.
 5. The wellhead system of claim 4, wherein the casing hanger has an upper end, a lower end connected to the casing, a radially inner surface extending axially from the upper end to the lower end of the casing hanger, and a radially outer surface extending axially from the upper end to the lower end of the casing hanger; wherein the outer surface of the casing hanger includes an annular recess configured to receive the first ram block and the second ram block.
 6. The wellhead system of claim 5, wherein the annular recess in the casing hanger is defined by a frustoconical shoulder disposed at an angle α relative to a radius of the central axis; wherein the first ram block sealingly engages the frustoconical shoulder in the advanced position; wherein the second ram block sealingly engages the frustoconical shoulder in the advanced position.
 7. The wellhead system of claim 6, wherein first and second ram blocks each further include an engagement surface disposed at an angle β relative to a radius of the central axis, which is configured to engage with the frustoconical shoulder of the annular recess when the first and second ram blocks are in the advanced position.
 8. The wellhead system of 7 wherein the angles α and β are each between 2 and 14°.
 9. The wellhead system of claim 1, wherein the first ram block has a pair of end faces and a semi-cylindrical surface extending circumferentially between the end faces of the first ram block; wherein the second ram block has a pair of end faces and a semi-cylindrical surface extending circumferentially between the end faces of the second ram block; wherein each end face of the first ram block opposes one end face of the second ram block; wherein each end face of the first ram block is configured to engage and interlock with one of the end face of the second ram block.
 10. The wellhead system of claim 1, further comprising: a first actuator coupled to the wellhead housing and configured to transition the first ram block between the withdrawn and advanced positions; and a second actuator coupled to the wellhead housing and configured to transition the second ram block between the withdrawn and advanced positions.
 11. The wellhead system of claim 1, further comprising: a second casing string disposed within the wellhead housing, wherein the second casing string includes a casing hanger and casing extending axially from the casing hanger; and a third ram block movably coupled to the housing and positioned axially above the first ram block; a fourth ram block movably coupled to the housing and positioned axially above the second ram block, wherein the fourth ram block is radially opposed to the third ram block; wherein the third ram block has a withdrawn position radially spaced apart from the casing hanger of the second casing string and an advanced position sealingly engaging the casing hanger of the second casing string; wherein the fourth ram block has a withdrawn position radially spaced apart from the casing hanger of the second casing string and an advanced position sealingly engaging the casing hanger of the second casing string.
 12. The wellhead casing of claim 11, wherein the casing hanger of the second casing string is seated on the casing hanger of the first casing string.
 13. A method of installing one or more casing string within a wellhead housing, the method comprising: (a) lowering a first casing string into a wellhead housing having a central axis, wherein the casing string includes a casing hanger and casing extending axially from the casing hanger; (b) moving a first ram block coupled to the wellhead housing radially inward into sealing engagement with the casing hanger; and (c) moving a second ram block coupled to the wellhead housing radially inward into sealing engagement with the casing hanger.
 14. The method of claim 13, further comprising: (d) sealing an annulus radially disposed between the wellhead housing and the casing hanger of the first casing string.
 15. The method of claim 13, wherein (b) comprises: (b1) moving the first ram block with a first linear actuator coupled to the wellhead housing; and (b2) moving the second ram block with a second linear actuator coupled to the wellhead housing.
 16. The method of claim 13, further comprising: (e) interlocking a pair end faces of the first ram block with a pair of opposed end faces of the second ram block.
 17. The method of claim 13, further comprising: pumping cement down the first casing string and up an annulus between the first casing string and the wellhead housing before (b) and (c).
 18. The method of claim 13, further comprising: seating the casing hanger of the first casing string against an annular shoulder on an inner surface of the wellhead housing.
 19. The method of claim 18, wherein (b) and (c) comprise: compressing the casing hanger axially downward into the annular shoulder.
 20. The method of claim 13, further comprising: (d) lowering a second casing string into the first casing string and the wellhead housing, wherein the casing string includes a casing hanger and casing extending axially from the casing hanger; (e) moving a third ram block coupled to the wellhead housing radially inward into sealing engagement with the casing hanger of the second casing string; and (f) moving a fourth ram block coupled to the wellhead housing radially inward into sealing engagement with the casing hanger of the second casing string.
 21. The method of claim 20, further comprising: pumping cement down the second casing string and up an annulus between the second casing string and the first casing string before (e) and (f).
 22. The method of claim 20, further comprising: seating the casing hanger of the second casing string against an upper end of the casing hanger of the first casing string.
 23. A wellhead system, comprising: a wellhead housing having a central axis, an upper end, and a radially inner surface extending axially from the upper end; a first annular recess extending radially outward from the inner surface; a first ram block disposed in the first annular recess; a second ram block disposed in the first annular recess axially opposite the first ram block; a first linear actuator coupled to the wellhead housing and configured to move the first ram block radially toward and away from the second ram block; a second linear actuator coupled to the wellhead housing and configured to move the second ram block radially toward and away from the first ram block.
 24. The wellhead system of claim 23, wherein the first annular recess is defined by an upper planar surface extending radially from the inner surface, a lower planar surface extending radially from the inner surface; and an annular surface extending axially from the upper planar surface to the lower planar surface.
 25. The wellhead system of claim 24, wherein a first annular seal assembly disposed along the upper annular surface and a second seal assembly is disposed along the lower annular surface; wherein each seal assembly engages the first and second ram blocks.
 26. The wellhead system of claim 23, further comprising: a second annular recess extending radially outward from the inner surface and axially spaced above the first annular recess; a third ram block disposed in the second annular recess; a fourth rain block disposed in the second annular recess axially opposite the third ram block; a third linear actuator coupled to the wellhead housing and configured to move the third ram block radially toward and away from the fourth ram block; a fourth linear actuator coupled to the wellhead housing and configured to move the fourth ram block radially toward and away from the third ram block. 